Starting with a feral, outbred population, we established a subline (ev- of mice (Mus musculus domesticus) completely devoid of any endogenous MuMTV sequences and another subline (ev-1) carrying only a single provirus related to the highly tumorigenic provirus derived from the milk of the inbred C3H mouse strain. As Aim #1 we plan to further expand and characterize these subpopulations of mice. Aim #2 is to determine in the ev- and ev-1 mice whether MuMTV genes are essential for spontaneous, chemical- or hormonal-induced mammary tumorigenesis and to compare the response to exogenous infection with MuMTV. Aim #3 is a longer-range goal designed to use the ev- mice as a sterile background for genetic crosses with inbred and wild mice to isolate individual MuMTV proviruses and determine their phenotype.